Inside a conventional tail female receptacle of light bulb series, the copper plates connected with conductor wires are usually placed into the grooves formed inside the housing of receptacles that have not a firm holding function to the copper plates so that the copper plates often have faltering phenomena, which in turn leads to an unstable contact between the copper plates and the copper blades of plugs resulting in a safety problem. furthermore, the connections between the conductor wires and tail receptacles simply rely on joining the conductor wires to the ends of copper plates. When in use, the tail receptacles are easily disconnected with the wires, resulting in potential dangers to people.